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https://github.com/angular/angular
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1ba9b7ac50
* Define `ResourceSnapshot<T>` as a type union of possible states for a
`Resource<T>`.
* Add `Resource.snapshot()` to convert a `Resource` to a signal of its
snapshot.
* Add `resourceFromSnapshots` to convert a reactive snapshot back into a
`Resource`.
By converting resources from/to `Signal<ResourceSnapshot>`s, full
composition of resources is now possible on top of signal composition APIs
like `computed` and `linkedSignal`.
For example, a common feature request is to have a `Resource` which retains
its value when its reactive source (params) changes. This can now be built
as a utility, leveraging `linkedSignal`'s previous value capability:
```ts
function withPreviousValue<T>(input: Resource<T>): Resource<T> {
const derived = linkedSignal({
source: input.snapshot,
computation: (snap, previous) => {
if (snap.status === 'loading' && previous?.value) {
// When the input resource enters loading state, we keep the value
// from its previous state, if any.
return {status: 'loading', value: previous.value.value};
}
// Otherwise we simply forward the state of the input resource.
return snap;
},
});
return resourceFromSnapshots(derived);
}
// In application code:
userId = input.required<number>();
user = withPreviousValue(httpResource(() => `/user/{this.userId()}`));
// if `userId()` switches, `user.value()` will keep the old value until
// the new one is ready!
```
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|---|---|---|
| .. | ||
| api.ts | ||
| from_snapshots.ts | ||
| index.ts | ||
| resource.ts | ||